1. Field of the Invention
The present invention relates to a cold trap.
2. Description of the Related Art
There is a cold trap provided between a vacuum chamber and a vacuum pump, which is referred to as an in-line type cold trap. A cold panel is placed in an exhaust passage connecting an exhaust port of the vacuum chamber and an intake port of the vacuum pump. The vacuum pump may be a turbo molecular pump. When the in-line type cold trap is used in combination with the turbo molecular pump, the cold trap is primarily used to pump water vapor.
Cold traps of in-line type are generally grouped into two subcategories depending on the thickness in the direction of the exhaust passage. One is a cold trap of thin type and the other is a cold trap of thick type. The cold trap of thin type is implemented by a planar cold panel such as a baffle or a perforated plate. The cold trap of thick type is implemented by, for example, a cold panel of a barrel form having a surface extending in the direction of the exhaust passage.
Assuming that the cold trap of thick type has the same diameter as the cold trap of thin type, the cold trap of thick type normally has a larger surface area. For this reason, the thick cold trap is more advantageous than the thin cold trap in terms of the pumping speed. Further, the thick cold trap is more advantageous in terms of the conductance of the exhaust passage. However, the thin cold trap is planar and so occupies a large portion of the cross section of the exhaust passage to decrease the conductance thereof. Therefore, as compared to the thin cold trap, the thick cold trap is more advantageous in terms of the pumping speed of the downstream vacuum pump (e.g., a turbo molecular pump).
A suitable application of the thin cold trap is found in a case where an installation space for the thin cold trap is small. For example, a vacuum pump may be required to be installed at a location displaced with respect to the exhaust port of the vacuum chamber in a transversal direction (i.e., in a direction perpendicular to the exhaust passage). In this case, the intake port flange of the vacuum pump is eccentric with respect to the exhaust port flange of the vacuum chamber so that a conversion flange for converting the transversal position is interposed between the exhaust port flange and the intake port flange. Since an inherently narrow space between the vacuum chamber and the vacuum pump is shared by the conversion flange and the cold trap, the installation space available for the cold trap is particularly small.